limiting monthly network traffic

[cls@truffula.sj.ca.us]

[This message has also been posted to linux.debian.user.]

I'm moving my colocated Debian server to a place with
high overage charges.  My network link will carry
about 200x my bandwidth allocation for the month.
The new bandwidth allocation is about 8x what I've been using.
But if I get slashdotted and have a 10x month, it will
cost too much.
What's the Debian way to monitor total network traffic
and throttle down if I get too near the monthly limit?

Thanks

Cameron